The present invention relates to a device in an offshore oil production installation for transferring a fluid between at least two floating supports such as a production vessel producing a gas-free product (dead oil) from live crude and a loading buoy (CALM buoy) from which oil tankers are filled with the product to be transported to land (onshore).
The production vessel having an acronym FPSO (Floating Production Storage Offloading) is generally anchored in a zone where the live crude is produced and is separated from the CALM buoy by several kilometers of the order of 1 to 3 kilometers.
The device for transferring the dead oil from the production vessel to the CALM buoy consists of at least one line known as an export line, one end of which is connected to the production vessel and the other end of which is connected to the CALM buoy. These export lines consist of a flexible pipe or rigid tube as described in API 17B, 17J and 5CT (American Petroleum Institute).
When the export line is built rigid, the connections at its ends are provided by kinds of ball joints (flex joints) so as to allow the export line to follow, on the other hand, the relative movements of each of the floating supports and, on the other hand, to more or less absorb the influences of the swell and marine currents likely to be present down to a certain depth in the sea. It is known from GB 2 335 723 to replace the conventional ball joint by flexible connecting means connecting the end of the rigid transport line to one of the floating supports and ensuring the continuity of flow of the crude between the two floating supports via the rigid transport line. However, according to this reference, the flexible pipe which replaces the conventional connection has the same dimensions as that connection, on order of a few meters. The rigid pipe stays partially submerged in a turbulent zone, and, consequently, the ends of the pipe undergo vibrations due to high marine currents. These vibrations in combination with the tensile forces cause early fatigue of the rigid pipe.
As the floating supports concerned can move independently of one another, and in any arbitrary direction, over a distance which is considered to be approximately equal to about 10% of the water depth of the sea on which the said supports are afloat, the amplitude of the relative movement between the two structures may thus be of the order of 20% of the said depth.
In order to allow these relative moments which may represent from 10 to 50% of the distance between the floating supports, it is common practice to provide an export line of a length much greater than the distance separating the two floating supports.
Furthermore, dynamic loadings in bending and vibrations are generated in the standing part of the export line by the movement of the swell, the marine current and the relative displacements of the supports. In addition, tensions are also created at the ends of the export line, these tensions being due mainly to the weight of the said export line.
The combination of the dynamic loadings, of the vibrations and of the tensions leads to significant fatigue of the export line at the end connections, which significantly reduces the life of the export line.
In the case of a rigid tube and in order to reduce vibration, the zones subjected to significant vibrations are equipped with additional special-purpose means such as anti-vibration strakes, for example. However, a solution such as this leads to additional cost of manufacture of the export line.
In order to reduce the tension caused by the weight of the line and to limit the tension at the ends, buoys with positive buoyancy have been widely used to create a single or double wave between the two floating supports. The series (of which there may be more than one) of buoys corresponding to the waves formed along the length of the export line gives the export line an additional length between its ends, which makes it possible to absorb the differences in length that are due to the relative displacements of the floating supports and for this to be possible under the most unfavourable operating conditions, that is to say when the said floating supports are moving in opposite directions.
One disadvantage of having buoys of positive buoyancy on the export line lies in the fact that the cost of the said export line is increased significantly without in any way solving the problems associated with the bending moments generated by the dynamic loadings or those associated with the vibrations caused by marine currents in particular.
In addition, by reducing the apparent weight of the export line, the latter tends to move with not insignificant amplitudes of movement as a function of the marine currents. These repeated movements lead to significant fatigue, mainly at the connections with the floating supports.
Another solution consists in laying the rigid export line on the seabed and in connecting its ends to the floating supports by risers. However, the length of such an installation is entirely prohibitive and cannot really be envisaged for great depths.
The object of the present invention is to overcome the aforementioned drawbacks by dissociating the bending moments developed by the movements of the floating supports and the vibrations from the tensile loadings developed by the weight of the export line.
The present invention concerns a device for transferring fluid between two floating supports at the surface of the sea, wherein the sea has a turbulent zone determined over a given depth. The device comprises a rigid hollow transport line which is submerged catenary-fashion in the sea. Flexible connecting means connect each end of the rigid transport line to one of the floating supports. The said connecting means ensure continuity of flow of the crude between the two floating supports via the rigid transport line. The entire rigid transport line including its ends is submerged in the sea to a depth greater than the turbulent zone.
What happens is that for a given region of the exploited oil field, the specialists can quite easily determine the height of the layer of water (turbulent zone) beneath which the movements of the swell are relatively small and in which the marine currents are weak, that is to say, in practice, a maximal speed of the marine currents less than 1 m/s or even 0.5 m/s. According to the invention, the rigid pipe is submerged within a non turbulent zone, defined by these speeds.
Each flexible and deformable connection connecting one end of the export line to the corresponding floating support absorbs all the dynamic bending stresses and vibrations without the need for additional special-purpose equipment.